The present invention relates to a process for obtaining formic acid by thermal separation of a stream comprising formic acid and a tertiary amine (I), which, at a pressure of 1013 hPa abs, has a boiling point which is at least 5° C. higher than that of formic acid, in which    a liquid stream comprising formic acid, methanol, water and tertiary amine (I) is produced by combining methyl formate, water and tertiary amine (I), from 10 to 100% by weight of the methanol comprised therein are separated off from the liquid stream obtained from step (a) and    formic acid is removed by distillation from the liquid stream comprising formic acid, water and tertiary amine (I) obtained from step (b) in a distillation apparatus at a temperature at the bottom of from 100 to 300° C. and a pressure of from 30 to 3000 hPa abs.
Formic acid is an important and versatile product. It is used, for example, for acidification in the production of animal feeds, as preservative, as disinfectant, as assistant in the textile and leather industry, as a mixture with its salts for deicing aircraft and runways and also as synthetic building block in the chemical industry.
The most widespread process at present for preparing formic acid is the hydrolysis of methyl formate which can be obtained, for example, from methanol and carbon monoxide. The aqueous formic acid obtained by hydrolysis is subsequently concentrated, for example using an extraction auxiliary such as a dialkylformamide (DE 25 45 658 A1).
In addition, obtaining formic acid by thermal dissociation of compounds of formic acid and a tertiary nitrogen base is also known. These compounds are generally acidic ammonium formates of tertiary nitrogen bases, in which the formic acid has reacted beyond the stage of classical salt formation with the tertiary nitrogen bases to form stable addition compounds bridged by hydrogen bonds. The addition compounds of formic acid and tertiary nitrogen bases can be formed by combining the tertiary nitrogen base and a formic acid source. Thus, for example, WO 2006/021,411 discloses the preparation of such addition compounds in general by (i) direct reaction of the tertiary nitrogen base with formic acid, (ii) by transition metal-catalyzed hydrogenation of carbon dioxide to formic acid in the presence of the tertiary nitrogen base, (iii) by reaction of methyl formate with water and subsequent extraction of the resulting formic acid by means of the tertiary nitrogen base and (iv) by reaction of methyl formate with water in the presence of the tertiary nitrogen base.
The general advantages of using addition compounds of formic acid and tertiary nitrogen bases for obtaining formic acid are that the addition compounds firstly bind the formic acid strongly enough to withdraw the formic acid from the medium, for example the reaction medium, in which the formic acid has been formed by chemical synthesis or, for example, from a dilute formic acid solution, and thereby allow the formic acid to be separated off more readily in the form of its addition compounds, but are weak enough for the formic acid subsequently to be able to be released again from the addition compounds by thermal dissociation in order to obtain it in concentrated and purified free form.
EP 0 001 432 A discloses a process for obtaining formic acid by hydrolysis of methyl formate in the presence of a tertiary amine, in particular an alkylimidazole, to form addition compounds of formic acid and the tertiary amine. The hydrolysis mixture obtained, which comprises unreacted methyl formate, water, methanol, addition compounds and tertiary amine, is freed of the low boilers methyl formate and methanol in a first distillation column. In a second column, the remaining bottom product is dewatered. The dewatered bottom product from the second column, which still comprises addition compounds and tertiary amine, is then fed to a third column and in this the addition compounds are thermally dissociated into formic acid and tertiary amine. The formic acid liberated is removed as overhead product. The tertiary amine collects in the liquid phase and is recirculated to the hydrolysis.
DE 34 28 319 A discloses a process for obtaining formic acid by hydrolysis of methyl formate. The hydrolysis mixture obtained, which comprises unreacted methyl formate, water, methanol and formic acid, is freed of the low boilers methyl formate and methanol in a first distillation column. The aqueous formic acid obtained at the bottom is subsequently extracted with a relatively high-boiling amine, in particular a relatively long-chain, hydrophobic C6-C14-trialkylamine, in the presence of an additional hydrophobic solvent, in particular an aliphatic, cycloaliphatic or aromatic hydrocarbon, and thereby converted into an aqueous addition compound of formic acid and the amine. This is dewatered in a second distillation column. The dewatered addition compound obtained at the bottom is then fed to a third distillation column and thermally dissociated therein. The hydrophobic solvent is present both in the overhead stream and the bottoms from the column. The gaseous overhead stream comprises mainly the formic acid liberated together with the hydrophobic solvent. This stream is liquefied again in the condenser. This results in formation of two phases, namely a polar formic acid phase and a hydrophobic solvent phase. The formic acid phase is discharged as product and the solvent phase is returned as runback to the column. Due to the presence of the hydrophobic solvent, complete dissociation of the adduct, which according to the teaching of the DE first publication occurs without decomposition of formic acid, can be achieved. The (virtually) formic acid-free bottoms comprise the hydrophobic amine and the hydrophobic solvent. This is recirculated to the extraction stage.
WO 2006/021,411 describes a process for obtaining formic acid by thermal dissociation of an addition compound of formic acid and a tertiary amine (quaternary ammonium formate), in which the tertiary amine has a boiling point of from 105 to 175° C. Alkylpyridines are mentioned as preferred tertiary amines. The specific boiling range of the tertiary amines increases the color stability of the formic acid obtained. The addition compound to be used can in general be obtained from the tertiary amine and a formic acid source. The output from the adduct synthesis is advantageously firstly freed of volatile constituents and then fed to the thermal dissociation. The thermal dissociation is carried out as usual in a distillation column. The formic acid liberated is removed as overhead product. The tertiary amine which may still comprise residues of formic acid collects in the liquid phase and can be recirculated to the formic acid source.
EP 0 563 831 A reports an improved process for the thermal dissociation of an addition compound of formic acid and a tertiary amine (quaternary ammonium formate) to give formic acid. The addition compound to be used can in general be obtained from the tertiary amine and a formic acid source. The output from the synthesis is advantageously firstly freed of volatile constituents and then fed to a distillation column for thermal dissociation. The improvement comprises essentially carrying out the thermal dissociation of the addition compound in the presence of a secondary formamide which increases the color stability of the formic acid obtained. The formic acid liberated is removed as overhead product. The tertiary amine and the secondary formamide collect in the liquid phase and can be recirculated to the formic acid source.
WO 2012/000,964 teaches a process for obtaining formic acid by thermal separation of a stream comprising formic acid and a tertiary amine, in which combining tertiary amine and a formic acid source produces a liquid stream comprising formic acid and a tertiary amine in a molar ratio of from 0.5 to 5, from 10 to 100% by weight of the secondary components comprised therein are separated off and formic acid is removed by distillation from the resulting liquid stream in a distillation apparatus at a temperature at the bottom of from 100 to 300° C. and a pressure of from 30 to 3000 hPa, and the bottom output from the distillation apparatus is separated into two liquid phases of which the upper liquid phase is enriched in tertiary amine and is recirculated to the formic acid source and the lower liquid phase is enriched in formic acid and is recirculated to removal of the secondary components and/or to the distillation apparatus.
EP application No. 11 194 619.0 teaches an improved process for obtaining formic acid by thermal separation of a stream comprising formic acid and a tertiary amine, in which a liquid stream comprising formic acid, tertiary amine and water is produced by combining tertiary amine and a formic acid source in the presence of water, water and organic degradation products of the tertiary amine are removed and formic acid is distilled from the resulting liquid stream in a distillation apparatus, where the stream comprising water and organic degradation products of the tertiary amine which has been separated off is separated into two liquid phases, the upper liquid phase is removed and the lower, water-comprising liquid phase is recirculated to the formic acid source. The process described in EP application No. 11 194 619.0 allows the relatively simple removal and discharge of various volatile by-products and degradation products of the tertiary amine.
EP application No. 11 194 607.5 teaches an improved process for obtaining formic acid by thermal separation of a stream comprising formic acid and a tertiary amine, in which a liquid stream comprising formic acid and tertiary amine is produced by combining tertiary amine and a formic acid source, secondary components comprised therein are separated off, formic acid is distilled off from the resulting liquid stream in a distillation apparatus, the bottom output from the distillation apparatus is separated into two liquid phases and the upper liquid phase is recirculated to the formic acid source and the lower liquid phase is recirculated to the removal of secondary components and/or the distillation apparatus, where low boilers are separated off by distillation from the upper liquid phase and the depleted stream is recirculated. The process described in EP application No. 11 194 607.5 allows the relatively simple removal and discharge of various volatile by-products and degradation products of the tertiary amine.
It is an object of the present invention to discover an improved process for obtaining formic acid by thermal separation of a stream comprising formic acid and a tertiary amine, which process has advantages over the prior art and is able to give formic acid in high yield and high concentration. In particular, the improved process should also function stably over long operating times and produce formic acid in constant high purity. The process should naturally be able to be carried out very simply and with a very low energy consumption.